An electromagnetic signal encodes information in high and low amplitude states or phase changes of a carrier wave of electromagnetic radiation. The electromagnetic signal can be transmitted over a waveguide, such as an optical fiber, or over free space. One way in which to generate an electromagnetic signal is to directly modulate the drive current of a laser or light-emitting diode (“LED”). This process of generating electromagnetic signals is called “direct modulation.” Unfortunately, direct modulation of radiation emitting devices has a number of drawbacks. First, the modulation rate averaged over power is limited, and second, high speed generated high and low amplitude states of the electromagnetic signal may be indistinguishable. In addition, direct modulation can distort analog signals and shift the output wavelength of an electromagnetic signal, an effect called “chirp,” which adds to chromatic dispersion.
The importance of these limitations depends on the system design and the distance over which the electromagnetic signals are transmitted. For example, when an electromagnetic signal is transmitted over many kilometers, these problems can occur with direct modulation data rates as low as 1 Gbit/s. On the other hand, when an electromagnetic signal is transmitted less than a kilometer or two, direct modulation may be sufficient at data rates as high as 10 Gbit/s.
In either case, when direct modulation fails to meet performance requirements, external modulators (i.e., modulators) can be used. A modulator can be operated to encode information in an electromagnetic signal by passing an unmodulated carrier wave of electromagnetic radiation through the modulator with the modulator operated to change the amplitude or phase of the carrier wave. Modulators can be operated at faster modulation rates than direct modulation of a laser or an LED, and typically do not alter the wavelength of the electromagnetic radiation. In recent years, the demand for faster and more efficient modulators has increased in order to keep pace with the increasing demand for high speed data transmission between communicating devices.